Process for removing water from a mixture containing water and zinc chloride

ABSTRACT

A process for the removal of water from a mixture comprising water and zinc chloride, which comprises adding to said mixture comprising water and zinc chloride an aprotic, polar diluent
         whose boiling point in the case where an azeotrope is not formed between said diluent and water under the pressure conditions of the distillation mentioned below is higher than the boiling point of water and which is in liquid form at this boiling point of water or   which forms an azeotrope or heteroazeotrope with water under the pressure and temperature conditions of the distillation mentioned below, and   distilling the mixture comprising water, zinc chloride and the diluent with removal of water or said azeotrope or said heteroazeotrope from this mixture, giving an anhydrous mixture comprising zinc chloride and said diluent.

RELATED APPLICATION

This application is a national stage application (under 35 U.S.C. 371)of PCT/EP2003/007149 filed Jul. 4, 2003 which claims benefit to Germanapplication Serial No. 102 31 296.6 filed Jul. 10, 2002.

The present invention relates to a process for the removal of water froma mixture comprising water and zinc chloride, which comprises

adding to said mixture comprising water and zinc chloride an aprotic,polar diluent

whose boiling point in the case where an azeotrope is not formed betweensaid diluent and water under the pressure conditions of the distillationmentioned below is higher than the boiling point of water and which isin liquid form at this boiling point of water or

which forms an azeotrope or heteroazeotrope with water under thepressure and temperature conditions of the distillation mentioned below,and

distilling the mixture comprising water, zinc chloride and the diluentwith removal of water or said azeotrope or said heteroazeotrope fromthis mixture, giving an anhydrous mixture comprising zinc chloride andsaid diluent.

Anhydrous zinc chloride or anhydrous mixtures comprising a liquiddiluent and zinc chloride are industrially important starting compoundswhich are used, inter alia, in electroplating technology in thezinc-plating of metallic materials or, owing to the Lewis acidproperties of the zinc chloride, as catalyst or catalyst constituent.

After these uses, the known problem arises of recovering the zincchloride from the mixture obtained after the use in a form such that itcan be re-used industrially, usually in the form of anhydrous zincchloride or anhydrous mixtures comprising a liquid diluent and zincchloride.

This recovery usually gives mixtures which comprise water as well aszinc chloride.

A particular difficulty consists in recovering anhydrous zinc chlorideor anhydrous mixtures comprising a liquid diluent and zinc chloride frommixtures of this type, as is known, for example, from Report ofInvestigations No. 9347, “Method for Recovering Anhydrous ZnCl₂ fromAqueous Solutions”, B. R. Eichbaum, L. E. Schultze, United StatesDepartment of the Interior, Bureau of Mines, 1991, pages 1-10, (“RI9347”).

Owing to the high solubility of zinc chloride in water, concentration ofaqueous zinc chloride solutions gives a highly viscous mother liquorwhich is very difficult to filter in order to remove precipitated zincchloride from the mother liquor.

Thus, RI 9347, page 4, Table 1, discloses that unknown by-products areobtained in addition to zinc chloride on drying of zinc chloride at 70°C./60 days or at 150° C./8 days. Furthermore, the drying times quotedare uneconomically long for recovery of zinc chloride.

According to RI 9347, page 4, Table 2, the attempt to obtain zincchloride by spray-drying a zinc chloride solution at 100° C. under airdoes not give a powder, but instead a moist slurry.

The literature cited in RI 9347 on page 2 and RI 9347 itself proposeliberating zinc chloride from an aqueous solution with addition ofammonia or ammonium chloride, giving a zinc diamine dichloride complex,from which zinc chloride is liberated.

As is evident from RI 9347, this process does not give a pure zincdiamine dichloride complex, but instead a zinc diamine dichloridecomplex which is contaminated with various oxy or hydroxy compounds ofzinc.

In addition, RI 9347 describes on page 8, right-hand column, that theliberation of zinc chloride from the zinc diamine dichloride complexdoes not take place to completion at temperatures up to 400° C.; attemperatures above 400° C., explosive mixtures are disadvantageouslyobtained, attributable to decomposition of the ammonia.

It is an object of the present invention to provide a process whichenables the removal of water from a mixture comprising water and zincchloride in a technically simple and economical manner.

We have found that this object is achieved by the process defined at theoutset.

In the process according to the invention, water is removed from amixture comprising water and zinc chloride.

The mixing ratio of water to zinc chloride in the starting mixture isnot crucial per se. With increasing ratio of zinc chloride to water, theviscosity of the mixture increases significantly, making handling of themixture increasingly complex.

A proportion of zinc chloride, based on the total weight of zincchloride and water, in the region of at least 0.01% by weight,preferably at least 0.1% by weight, particularly preferably at least0.25% by weight, especially preferably at least 0.5% by weight, hasproven advantageous.

A proportion of zinc chloride, based on the total weight of zincchloride and water, in the region of at most 60% by weight, preferablyat most 35% by weight, particularly preferably at most 30% by weight,has proven advantageous.

The starting mixture can consist of zinc chloride and water.

Besides zinc chloride and water, the starting mixture may comprisefurther constituents, such as ionic or nonionic, organic or inorganiccompounds, in particular those which are homogeneously miscible with thestarting mixture to form a single phase or are soluble in the startingmixture.

In a preferred embodiment, it is possible to add an inorganic or organicacid. Preference is given to the use of acids which have a boiling pointunder the distillation conditions in the process according to theinvention which is lower than the boiling point of the aprotic, polardiluent. Particular preference is given to hydrohalic acids, such as HF,HCl, HBr or HI, in particular HCl.

The amount of acid can advantageously be selected in such a way that thepH of the mixture comprising water and zinc chloride is less than 7.

The amount of acid can advantageously be selected in such a way that thepH of the mixture comprising water and zinc chloride is greater than orequal to 0, preferably greater than or equal to 1.

Starting mixtures of this type can advantageously be obtained byextraction with a water-containing extractant, in particular with water,of a reaction mixture which has been obtained in the hydrocyanation ofpentenenitrile in the presence of a catalyst system comprising Ni(0),one or more phosphorus-containing ligands and zinc chloride, to giveadiponitrile.

The preparation of a reaction mixture of this type is known per se, forexample from U.S. Pat. No. 4,705,881. According to U.S. Pat. No.3,773,809, the catalyst can be removed from the reaction mixture byextraction, for example with cyclohexane, with the zinc chloride thatremains in the product stream comprising the predominant proportion ofthe adiponitrile. In order to obtain pure adiponitrile, the zincchloride can be removed from a product stream of this type in a mannerknown per se by reaction with ammonia, as described, for example, inU.S. Pat. No. 3,766,241.

The extraction can advantageously be carried out under conditions underwhich the extractant and the reaction mixture are in the form of twophases.

If water is employed as extractant, temperatures of at least 0° C.,preferably at least 5° C., in particular at least 30° C., have provenadvantageous.

If water is employed as extractant, temperatures of at most 200° C.,preferably at most 100° C., in particular at most 50° C., have provenadvantageous.

This gives rise to pressures in the range from 10⁻³ to 10 MPa,preferably from 10⁻² to 1 MPa, in particular from 5*10⁻² to 5*10⁻¹ MPa.

The phase separation can be carried out in a manner known per se inapparatuses described for such purposes, as are known, for example,from: Ullmann's Encyclopedia of Industrial Chemistry, Vol. B3, 5th Edn.,VCH Verlagsgesellschaft, Weinheim, 1988, pages 6-14 to 6-22.

The optimum apparatuses and process conditions for phase separation caneasily be determined here through some simple preliminary experiments.

In accordance with the invention, an aprotic, polar diluent whoseboiling point under the pressure conditions of the distillationmentioned below is higher than the boiling point of water and which isin liquid form at this boiling point of water is added to said mixturecomprising water and zinc chloride. The addition of water to saidmixture can be carried out before the distillation or during thedistillation.

The pressure conditions for the subsequent distillation are not crucialper se. Pressures of at least 10⁻⁴ MPa, preferably at least 10⁻³ MPa, inparticular at least 5*10⁻³ MPa, have proven advantageous.

Pressures of at most 1 MPa, preferably at most 5*10⁻¹ MPa, in particularat most 1.5*10⁻¹ MPa, have proven advantageous.

Depending on the pressure conditions and the composition of the mixtureto be distilled, the distillation temperature then becomes established.

At this temperature, the aprotic, polar diluent is, in accordance withthe invention, in liquid form. For the purposes of the presentinvention, the term aprotic, polar diluent is taken to mean both asingle diluent and also a mixture of diluents of this type, where, inthe case of a mixture of this type, said physical properties accordingto the invention relate to this mixture.

Furthermore, the aprotic, polar diluent has, in accordance with theinvention, a boiling point under these pressure and temperatureconditions which, in the case where an azeotrope is not formed betweenthe diluent and water, is higher than that of water, preferably by atleast 5° C., in particular at least 20° C. and preferably at most 200°C., in particular at most 100° C.

Aprotic, polar organic and inorganic diluents are known per se, forexample from: Jerry March, Advanced Organic Chemistry, 2nd Edn.,McGraw-Hill, International Student Edition, Hamburg, 8th reprint (1984),1977, pp. 331-336; Organikum, 2nd reprint of the 15th Edition, VEBDeutscher Verlag der Wissenschaften, Berlin, 1981, pp. 226-227;Streitwieser/Heathcock, Organische Chemie, Verlag Chemie, Weinheim,1980, p. 172.

Suitable are, for example, amides, in particular dialkylamides, such asdimethylformamide, dimethylacetamide, N,N-dimethylethyleneurea (DMEU),N,N-dimethylpropyleneurea (DMPU), hexamethylenephosphoric triamide(HMPT), ketones, sulfur-oxygen compounds, such as dimethyl sulfoxide,tetrahydrothiophene 1,1-dioxide, nitro aromatic compounds, such asnitrobenzene, nitroalkanes, such as nitromethane and nitroethane,ethers, such as diethers of diethylene glycol, for example diethyleneglycol dimethyl ether, alkylene carbonates, such as ethylene carbonate,nitriles, such as acetonitrile, propionitrile, n-butyronitrile,n-valeronitrile, cyanocyclopropane, acrylonitrile, crotonitrile, allylcyanide and pentenenitriles.

Aprotic, polar diluents of this type can be employed alone or in theform of a mixture.

Aprotic, polar diluents of this type may comprise further diluents,preferably aromatic compounds, such as benzene, toluene, o-xylene,m-xylene or p-xylene, aliphatic compounds, in particular cycloaliphaticcompounds, such as cyclohexane or methylcyclohexane, or mixturesthereof.

In a preferred embodiment, it is possible to employ diluents which forman azeotrope or heteroazeotrope with water. The amount of diluentcompared with the amount of water in the mixture is not crucial per se.It is advantageous to employ more liquid diluent than corresponds to theamount to be distilled off through the azeotropes, so that excessdiluent remains as bottom product.

If use is made of a diluent which does not form an azeotrope with water,the amount of diluent compared with the amount of water in the mixtureis not crucial per se.

Organic diluents are advantageously suitable, preferably those having atleast one nitrile group, in particular one nitrile group.

In a preferred embodiment, the nitrile employed can be a saturatedaliphatic nitrile or an olefinically unsaturated aliphatic nitrile.Particularly suitable are nitriles having 3, 4, 5, 6, 7, 8, 9 or 10, inparticular 4, carbon atoms, calculated without the nitrile groups,preferably the nitrile group.

In a particularly preferred embodiment, the diluent employed can be anolefinically unsaturated aliphatic mononitrile selected from the groupconsisting of 2-cis-pentenenitrile, 2-trans-pentenenitrile,3-cis-pentenenitrile, 3-trans-pentenenitrile, 4-pentenenitrile,E-2-methyl-2-butenenitrile, Z-2-methyl-2-butenenitrile,2-methyl-3-butenenitrile or a mixture thereof.

2-cis-Pentenenitrile, 2-trans-pentenenitrile, 3-cis-pentenenitrile,3-trans-pentenenitrile, 4-pentenenitrile, E-2-methyl-2-butenenitrile,Z-2-methyl-2-butenenitrile, 2-methyl-3-butenenitrile and mixturesthereof are known and can be obtained by processes known per se, such asby hydrocyanation of butadiene in the presence of catalysts, for exampleas described in U.S. Pat. No. 3,496,215, or the linear pentenenitrilesby isomerization of 2-methyl-3-butenenitrile as described in WO97/23446.

Particularly advantageous here are mixtures of said pentenenitrileswhich comprise 2-cis-pentenenitrile, 2-trans-pentenenitrile or mixturesthereof mixed with 3-cis-pentenenitrile, 3-trans-pentenenitrile,4-pentenenitrile, E-2-methyl-2-butenenitrile,Z-2-methyl-2-butenenitrile, 2-methyl-3-butenenitrile or mixturesthereof. In mixtures of this type, a reduction in the concentration of2-cis-pentenenitrile, 2-trans-pentenenitrile,E-2-methyl-2-butenenitrile, Z-2-methyl-2-butenenitrile,2-methyl-3-butenenitrile or mixtures thereof takes place during thesubsequent distillation since these form azeotropes with water whichhave a lower boiling point than the azeotropes of 3-cis-pentenenitrile,3-trans-pentenenitrile, 4-pentenenitrile or mixtures thereof with water.In this embodiment, a mixture comprising 3-cis-pentenenitrile,3-trans-pentenenitrile, 4-pentenenitrile or mixtures thereof andanhydrous zinc chloride is obtained after the distillation as product ofthe process according to the invention.

This product can advantageously be employed for further hydrocyanationin the presence of a catalyst to give adiponitrile. A reduction in theconcentration of 2-cis-pentenenitrile, 2-trans-pentenenitrile,E-2-methyl-2-butenenitrile, Z-2-methyl-2-butenenitrile or2-methyl-3-butenenitrile is advantageous in as much as these twocompounds undergo said hydrocyanation to a considerably lesser extentthan 3-cis-pentenenitrile, 3-trans-pentenenitrile, 4-pentenenitrile ormixtures thereof.

If the diluent employed is 2-cis-pentenenitrile, 2-trans-pentenenitrile,3-cis-pentenenitrile, 3-trans-pentenenitrile, 4-pentenenitrile,E-2-methyl-2-butenenitrile, Z-2-methyl-2-butenenitrile,2-methyl-3-butenenitrile or mixtures thereof, mixing ratios ofpentenenitrile to the zinc chloride of at least 0.5 mol/mol, preferablyat least 5 mol/mol, particularly preferably at least 15 mol/mol, haveproven advantageous.

If the diluent employed is 2-cis-pentenenitrile, 2-trans-pentenenitrile,3-cis-pentenenitrile, 3-trans-pentenenitrile, 4-pentenenitrile,E-2-methyl-2-butenenitrile, Z-2-methyl-2-butenenitrile,2-methyl-3-butenenitrile or mixtures thereof, mixing ratios ofpentenenitrile to the zinc chloride of at most 10,000 mol/mol,preferably at most 5000 mol/mol, particularly preferably at least 2000mol/mol, have proven advantageous.

In accordance with the invention, the mixture comprising water, zincchloride and the diluent is distilled with removal of water from thismixture, giving an anhydrous mixture comprising zinc chloride and saiddiluent.

In the case of pentenenitrile as diluent, the distillation canadvantageously be carried out at a pressure of at most 200 kPa,preferably at most 100 kPa, in particular at most 50 kPa, andparticularly preferably at most 20 kPa.

In the case of pentenenitrile as diluent, the distillation canadvantageously be carried out at a pressure of at least 1 kPa,preferably at least 5 kPa, particularly preferably at 10 kPa.

The distillation can advantageously be carried out by one-stepevaporation, preferably by fractional distillation in one or more, suchas 2 or 3, distillation apparatuses.

The distillation can be carried out in apparatuses which areconventional for this purpose, as described, for example, inKirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, JohnWiley & Sons, New York, 1979, pages 870-881, such as sieve-tray columns,bubble-tray columns, packed columns, columns with side take-off ordividing-wall columns.

The distillation can be carried out batchwise.

The distillation can be carried out continuously.

For the purposes of the present invention, the term anhydrous zincchloride is taken to mean zinc chloride having a water content, based onthe total weight of zinc chloride and water, of less than 500 ppm byweight, preferably less than 50 ppm by weight and at least equal to 0ppm by weight.

EXAMPLES

The % by weight or ppm by weight data given in the examples are based,unless stated otherwise, on the total weight of the respective mixture.

The Zn or zinc chloride content was determined by atomic emissionspectrometry.

The chlorine content was determined by the Schoeniger method.

The water concentration was determined potentiometrically by titrationby the Karl-Fischer method.

Example 1

In a continuously operated vacuum distillation column with metal meshpacking (type CY, Sulzer Chemtech, internal diameter Ø=50 mm, height 130cm) with a thin-film evaporator as heat exchanger at the column bottom,a condenser operated at 30° C. at the top and a phase separation vesselcooled to 0° C. in the reflux, 240 g/h of a solution of 30% by weight ofzinc chloride in trans-3-pentenenitrile having a water content of 0.4%by weight were metered into the distillation column above the meshpacking. At a pressure of p=10 kPa (absolute), a two-phase mixture wasobtained as condenser distillate at 344 K. The upper phase, essentiallyconsisting of trans-3-pentenenitrile, was fed back continuously to thetop of the column. The lower phase essentially consisted of water andwas continuously pumped out of the phase separation vessel. Ahomogeneous solution of ZnCl₂ in trans-3-pentenenitrile was separatedoff at 348 K at the bottom of the column. The water content in thebottom product had dropped to 76 ppm by weight of H₂O after adistillation run time of 17 hours and to 50 ppm by weight after 41hours.

Example 2

1 kg of trans-3-pentenenitrile and 500 g of water were added to 4 kg ofthe bottom product obtained in Example 1. The homogeneous mixture wasmetered into the distillation column operated as in Example 1 at ametering rate of 206 g/h.

After continuous operation for 24 hours, the bottom product comprised350 ppm by weight of water, 16.9% by weight of chlorine, calculated asCl, and 15.5% by weight of Zn, in each case based on the total weight ofthe solution; an experimentally found Cl:Zn ratio of 2.01 can be derivedtherefrom.

Gas-chromatographic analysis by derivatization with MSTFA(2,2,2-trifluoro-N-methyl-N-(trimethylsilyl)acetamide) showed nodetectable quantities of the saponification product 3-pentenoic acid.

Analysis for polymeric degradation products by gel permeationchromatography showed no detectable quantities of polymeric product.

The zinc chloride solution in 3-pentenenitrile obtained in this way canbe employed in the hydrocyanation of 3-pentenenitrile in the presence ofnickel(0) phosphite catalysts and shows no difference in activitycompared with a solution freshly prepared from 3-pentenenitrile andanhydrous zinc chloride.

1. A process for the removal of water from a mixture comprising waterand zinc chloride, which comprises adding to said mixture comprisingwater and zinc chloride an aprotic, polar diluent whose boiling point inthe case where an azeotrope is not formed between said diluent and wateris higher than the boiling point of water and which is in liquid form atthis boiling point of water or which forms an azeotrope orheteroazeotrope with water, and distilling the mixture comprising water,zinc chloride and the diluent with removal of water or said azeotrope orsaid heteroazeotrope from this mixture, giving an anhydrous mixturecomprising zinc chloride and said diluent, wherein the aprotic, polardiluent employed is an aliphatic, olefinically unsaturated nitrileselected from the group consisting of 2-cis-pentenenitrile,2-trans-pentenenitrile, 3-cis-pentenenitrile, 3-trans-pentenenitrile,4-pentenenitrile, E-2-methyl-2-butenenitrile,Z-2-methyl-2-butenenitrile, 2-methyl-3-butenenitrile or a mixturethereof.
 2. A process as claimed in claim 1, wherein the diluent is ableto form an azeotrope or heteroazeotrope with water under thedistillation conditions.
 3. A process as claimed in claim 1, wherein themixture comprising water and zinc chloride has a pH of less than
 7. 4. Aprocess as claimed in claim 1, wherein the mixture comprising water andzinc chloride has a pH in the range from 0 to less than
 7. 5. A processas claimed in claim 1, wherein an acid is added to the mixturecomprising water and zinc chloride.
 6. A process as claimed in claim 5,wherein the acid employed is HCl.
 7. A process as claimed in claim 2,wherein the mixture comprising water and zinc chloride has a pH of lessthan
 7. 8. A process as claimed in claim 7, wherein the mixturecomprising water and zinc chloride has a pH in the range from 0 to lessthan
 7. 9. A process as claimed in claim 8, wherein an acid is added tothe mixture comprising water and zinc chloride.
 10. A process as claimedin claim 9, wherein the acid employed is HCl.
 11. A process as claimedin claim 1, wherein a proportion of zinc chloride, based on the totalweight of zinc chloride and water, in the amount is at least 0.01% byweight.
 12. A process as claimed in claim 1, wherein a proportion ofzinc chloride, based on the total weight of zinc chloride and water, inthe amount is at least 0.1% by weight up to 60% by weight.
 13. A processas claimed in claim 1, wherein a proportion of zinc chloride, based onthe total weight of zinc chloride and water, in the amount is at least0.5% by weight up to 30% by weight.
 14. The process as claimed in claim1, wherein the extraction of water occurs at a temperature of 0° C. to200° C.
 15. The process as claimed in claim 1, wherein the extraction ofwater occurs at a temperature from 5° C. to 100° C.
 16. The process asclaimed in claim 1, wherein the extraction of water occurs at atemperature from 30° C. to 50° C.
 17. The process as claimed in claim 1,wherein the extraction of water occurs at a pressure the range from 10⁻³to 10 MPa.
 18. The process as claimed in claim 14, wherein theextraction of water occurs at a pressure the range from 10⁻³ to 10 MPa.19. The process as claimed in claim 15, wherein the extraction of wateroccurs at a pressure the range from 10⁻² to 1 MPa.
 20. The process asclaimed in claim 16, wherein the extraction of water occurs at apressure the range from 5×10⁻² to 5×10⁻¹ MPa.